Isolates service instances and interface requests among them according to user identity.

Tracks running service instances and exposes privileged APIs for querying system state.

The Service Manager presents a series of Mojo interfaces to services, though in practice most interaction with the Service Manager is made simpler by using its corresponding C++ client library.

Mojo Recap

The Mojo system provides two key components of interest here - a lightweight message pipe concept allowing two endpoints to communicate, and a bindings layer that allows interfaces to be described to bind to those endpoints, with ergonomic bindings for languages used in Chrome.

Mojo message pipes are designed to be lightweight and may be read from/written to and passed around from one process to another. In most situations a developer won't interact with the pipes directly, but rather with bindings types generated to encapsulate a bound interface. To use the bindings, a developer defines their interface in the Mojom IDL format. With some build magic, the generated definitions can then be referenced from C++, JavaScript and Java code.

See the Mojo documentation for a complete overview, detailed explanations, and API references.

Services

A service is a collection of one or more private implementations of public Mojo interfaces which are reachable via the Service Manager. Every service is comprised of the following pieces:

A set of public Mojo interface definitions

A service manifest declarating arbitrarily named capabilities which are each comprised of one or more exposed Mojo interfaces.

Private implementation code which responds to lifecycle events and incoming interface requests, all driven by the Service Manager.

The Service Manager is responsible for starting new service instances on-demand, and a given service may have any number of concurrently running instances. The Service Manager disambiguates service instances by their unique identity. A service's identity is represented by the 3-tuple of its service name, user ID, and instance qualifier:

The service name is a free-form -- typically short -- string identifying the the specific service being run in the instance.

The user ID is a GUID string representing the identity of a user in the system. Every running service instance is associated with a specific user ID.

Finally, the instance qualifier is an arbitrary free-form string used to disambiguate multiple instances of a service for the same user.

As long as a service instance is running it must maintain an implementation of the service_manager.mojom.Service interface. Typically this is done in C++ code by implementing the C++ client library's service_manager::Service interface. This interface is driven by messages from the Service Manager and is used to receive incoming interface requests the Service Manager brokers from other services.

Every service instance also has an outgoing link back to the Service Manager which it can use to make interface requests to other services in the system. This is the service_manager.mojom.Connector interface, and it‘s commonly used via the C++ client library’s service_manager::Connector class.

A Simple Service Example

This section walks through the creation of a simple skeleton service.

Private Implementation

Consider this implementation of the service_manager::Service interface:

Main Entry Point

While services do not need to define a main entry point -- e.g. they may only intend to be embedded in other running processes -- for the sake of completeness we also define a ServiceMain definition so that the service can be run in its own process:

Building the my_service target produces a my_service.service (or on Windows, my_service.service.exe) binary in the output directory. This can be run as a standalone executable, but it will exit immediately without doing anything interesting, because it won't have a Service pipe to drive it. The Service Manager knows how to provide such a pipe when launching a service executable.

This service doesn‘t do much of anything. It will simply run forever (or at least until the Service Manager itself shuts down), ignoring all incoming messages. Before we expand on the definition of this service, let’s look at some of the details of the service_manager::Service interface.

OnStart

The Service implementation is guaranteed to receive a single OnStart() invocation from the Service Manager before anything else hapens. Once this method is called, the implementation can access its service_manager::ServiceContext via context(). This object itself exposes a few values:

service_info() is a service_manager::ServiceInfo structure describing the running service from the Service Manager‘s perspective. This includes the service_manager::Identity which uniquely identifies the running instance, as well as the service_manager::InterfaceProviderSpec describing the capability specifications outlined in the service’s manifest.

identity() is a shortcut to the Identity stored in the ServiceInfo.

connector() is a service_manager::Connector which can be used to make outgoing interface requests to other services.

For example, we could modify MyService to connect out to logger service on startup:

OnBindInterface

The OnBindInterface method on service_manager::Service is invoked by the Service Manager any time another service instance uses its own Connector to request an interface from this my_service instance. The Service Manager only invokes this method once it has already validated that the request meets the mutual constraints specified in each involved service's manifest.

The arguments to OnBindInterface are as follows:

remote_info is the service_manager::ServiceInfo corresponding to the remote service which is requesting this interface. The information in this structure is provided authoritatively by the Service Manager and can be trusted in any context.

interface_name is the (std::string) name of the interface being requested by the remote service. The Service Manager has already validated that the remote service requires at least one capability which exposes this interface from the local service.

handle is the mojo::ScopedMessagePipeHandle of an interface pipe which the remote service expects us to bind to a concrete implementation of the requested interface.

The Service Manager client library provides a service_manager::BinderRegistry class definition which can make it easier for services to bind incoming interface requests. Typesafe binding callbacks are added to an BinderRegistry ahead of time, and the incoming arguments to OnBindInterface can be forwarded to the registry, which will bind the message pipe if it knows how. For example, we could modify our MyService implementation as follows:

namespace{voidBindDatabase(my_service::mojom::DatabaseRequest request){
mojo::MakeStrongBinding(std::make_unique<my_service::DatabaseImpl>(),
std::move(request));}}// namespaceMyService::MyService(){// Imagine |registry_| is added as a member of MyService, with type// service_manager::BinderRegistry.// The |my_service::mojom::Database| interface type is inferred by the// compiler in the AddInterface call, and this effectively adds the bound// function to an internal map keyed on the interface name, i.e.// "my_service::mojom::Database" in this case.
registry_.AddInterface(base::Bind(&BindDatabase));}voidMyService::OnBindInterface(const service_manager::ServiceInfo& remote_info,const std::string& interface_name,
mojo::ScopedMessagePipeHandle handle){
registry_.BindInterface(interface_name, std::move(handle));}

Service Manifests

If some service were to come along and attempt to connect to my_service and bind the my_service::mojom::Database interface, we might see the Service Manager spit out an error log complaining that InterfaceProviderSpec prevented a connection to my_service.

In order for the interface to be reachable by other services, we must first fix its manifest's interface provider spec. The interface provider spec is a dictionary keyed by interface provider name, with each value representing the capability spec for that provider.

Each capability spec defines an optional "provides" key and an optional "requires" key.

The provides key value is a dictionary which is itself keyed by arbitrary free-form strings (capability names, implicitly scoped to the manifest's own service) whose values are lists of Mojom interface names exposed as part of that capability.

The requires key value is also a dictionary, but it's one which is keyed by remote service name. Each value is a list of capabilities required from the corresponding remote service.

Finally, every interface provider spec (often exclusively) contains one standard capability spec named “service_manager:connector”. This is the capability spec enforced when inter-service connections are made from a service's Connector interface.

This means that my_service has defined a database capability comprised solely of the my_service::mojom::Database interface. Any service which requires this capability can bind that interface from my_service.

The Service Manager will approve of the request and forward it on to the my_service instance's OnBindInterface method.

Testing

Now that we‘ve built a simple service it’s time to write a test for it. The Service Manager client library provides a test fixture base class in service_manager::test::ServiceTest that makes writing service integration tests straightforward. This test fixture runs an in-process Service Manager on a background thread which allows test service instances to be injected at runtime.

Let's look at a simple test of our service:

//services/my_service/my_service_unittest.cc

#include"base/bind.h"#include"base/run_loop.h"#include"services/service_manager/public/cpp/service_test.h"#include"path/to/some_interface.mojom.h"classMyServiceTest:public service_manager::test::ServiceTest{public:// Our tests run as service instances themselves. In this case each instance// identifies as the service named "my_service_unittests".MyServiceTest(): service_manager::test::ServiceTest("my_service_unittests"){}~MyServiceTest() override {}}
TEST_F(MyServiceTest,Basic){
my_service::mojom::DatabasePtr database;
connector()->BindInterface("my_service",&database);
base::RunLoop loop;// This assumes DropTable expects a response with no arguments. When the// response is received, the RunLoop is quit.
database->DropTable("foo", loop.QuitClosure());
loop.Run();}

If adding a new test binary for these tests, we can augment our BUILD.gn to use the service_test GN template like so:

Alright, there‘s a lot going on here. First we also have to create a service manifest for the test service itself, as the Service Manager needs to be able to reason about the test’s own required capabilities with respect to the service-under-test.

You may also notice that we have suddenly introduced a catalog in the service_test target incantation. Any runtime environment which hosts a Service Manager must provide the Service Manager implementation with a catalog of service manifests. This catalog defines the complete set of services recognized by the Service Manager instance and can be used in all kinds of interesting ways to control how various services are started in the system. See Service Manager Catalogs for more information.

For now let's just accept that we have to create a catalog rule for our test suite and plug it into the service_test target.

In practice, we typically try to avoid introducing new unittest binaries for individual services. Instead we have an aggregate service_unittests target defined in //services/BUILD.gn. There are several examples of other services adding their service tests to this suite.

Service Manager Catalogs

A catalog is an aggregation of service manifests which comprises a complete runtime configuration of the Service Manager.

The GN catalog target template defined in //services/catalog/public/tools/catalog.gni. provides a simple means of aggregating service manifests into a single build artifact. See the comments on the template for detailed documentation.

This GNI also defines a catalog_cpp_source target which can generate a static C++ representation of an aggregated catalog manifest so that it can be passed the Service Manager at runtime.

In general, service developers should never be concerned with creating new catalogs or instantiating the Service Manager, but it's important to be aware of these concepts. When introducing a new service into any runtime environment -- including Chrome, Content, or various unit test suites such as service_unittests discussed in the previous section -- your service manifest must be added to the catalog used in that environment.